A key attribute of the immune system is the discrimination between self and non-self antigens. Surface recognition molecules on lymphocytes are sufficiently diverse for any antigen to find some lymphoid cells with surface antibody or T-cell receptor whose binding constant for antigen is high enough to lead to activation, thereby inducing the clonal expansion of cells recognizing that antigen. When these activated cell types (B and T cells) synthesize molecules (antibodies, T cell receptors) that react with self antigens, they are eliminated from the body. Immunologists refer to the inability to produce substances that react with self antigens as tolerance. The mechanisms that give rise to tolerance are poorly understood but involve rendering unreactive the clones of both B and T cells that would otherwise carry out anti-self reactions.
The immune system in mice becomes responsive to foreign antigens in the days just after birth. All antigens present in the body at the time the system matures are considered by the immune system to be self. Thus if foreign antigens are incorporated into a newborn mouse, subsequent challenge with the same antigen will not induce an immune response. Because new clones of lymphocytes appear in an animal throughout its life, the tolerance to self must be an active, ongoing process. Suppressor T lymphocytes are thought to play an important role in maintaining tolerance by specifically suppressing lymphocyte clones that react against self-antigens.
The consequences of the failure of tolerance can be severe. It is well known that certain autoimmune diseases are associated with autoantibodies directed against hormones and cell surface antigens. Examples of these diseases and associated antigens include the following:
______________________________________ AUTOINMUNE DISEASE ASSOCIATED ANTIGEN ______________________________________ Diabetes insulin, insulin receptor, glutamate decarboxylase Myasthenia gravis acetylcholine receptor Autoimmune thyroiditis thyroglobulin, thyroid peroxidase Systemic lupus erythmatosus small nuclear RNA, DNA and (SLE) histones, phospholipids Pernicious anemia gastric parietal cell associated antigens Rheumatoid arthritis collagen, IgG Wegener's Granuloma proteinase 3 Biliary cirrhosis pyruvate dehydrogenase ______________________________________
Related diseases, in the sense of having autoimmune disease-like components or mechanisms, include asthma and acquired immunodeficiency syndrome. Relevant antigens recognized by antibodies in these disorders are VIP (asthma), and neurolikin, VIP, HLA antigens and DNA (HIV).
Tissue injury in autoimmune disease occurs by several humoral and cell-mediated mechanisms. Autoantibodies alone can cause certain diseases, e.g., myasthenia gravis and Graves disease, as demonstrated by adoptive transfer via administration of antibodies in animal models. Rose, N. R. and I. R. Mackay, Editors. The Autoimmune Diseases.Orlando: Academic Press, Inc., 1985. In other diseases, tissue damage is believed to occur by a combination of autoantibodies, lymphocyte and macrophage infiltration, and the release of inflammatory mediators, including proteases.
Antibodies can recognize small arrays of atoms as well as large epitopes composed of as many as 25 amino acid residues. The target molecule for an antibody could be a small hapten or a macromolecule like a protein or nucleic acid.
Antigen recognition by antibodies occurs at complementarity determining regions (CDRs) encoded by V (variable), D (diversity) and J (joining) genes. There are three CDRs each in the variable regions of H- and L-chains (V.sub.H, V.sub.L). The immune system possesses the ability to diversify V.sub.H and V.sub.L sequences, permitting generation of 10.sup.10 -10.sup.12 antibody combining sites. Diversification occurs by V--D--J gene rearrangements, mutations in V-genes (mutation rate 10.sup.-3 -10.sup.-4 /base pair/cell division.sup.1), and combinatorial variation due to pairing of individual V.sub.L and V.sub.H domains from the available genes (&gt;300 and &gt;1000, respectively). This sequence diversification allows for the evolution of new functions in antibodies, including catalysis. Homologies between trypsin, in particular the sequence surrounding the active site serine, with antibody L-chains are described. Erhan, A. and L. D. Greller, Do immunoglobulins have proteolytic activity? Nature. 251: 353-355, 1974. Efficient proteolysis by human autoantibodies to the bronchodilator peptide vasoactive intestinal peptide (VIP) has been observed (Paul, S., D. J. Volle, C. M. Beach, D. R. Johnson, M. J. Powell, and R. J. Massey, Catalytic hydrolysis of vasoactive intestinal peptide by human autoantibody, Science, 244: 1158-1162, 1989; and Paul, S., M. Sun, R. Mody, S. H. Eklund, C. M. Beach, R. J. Massey, and F. Hamel, Cleavage of vasoactive intestinal peptide at multiple sites by autoantibodies, J. Biol. Chem., 266: 16128-16134, 1991), as well as with monoclonal antibodies (Paul, S., M. Sun, R. Mody, H. K. Tewary, and A. Tramontano, Peptidolytic monoclonal antibody elicited by a neuropeptide, J. Biol. Chem., 267: 13142-13145, 1992) and a recombinant antibody light chain raised by immunization with VIP. Gao, Q., M. Sun, S. Tyutyulkova, A. Tramontano, R. J. Massey, and S. Paul, Substrate-driven formation of a proteolytic antibody light chain, Abstract presented at New York Academy of Sciences Conference on Immunoglobulin Gene Expression in Development and Disease, Montreal, Canada, Jul. 13-17, 1994. An autoantibody to thyroglobulin has also been shown to cleave thyroglobulin. Paul, S., L. Li, S. Tyutyulkova, M. D. Kazatchkine, and S. Kaveri, Catalytic activity of anti-thyroglobulin antibodies, Abstract presented at New York Academy of Sciences Conference on Immunoglobulin Gene Expression in Development and Disease, Montreal, Canada, Jul. 13-17, 1994. Autoantibody catalyzed hydrolysis of VIP has been independently reproduced (Suzuki, H., H. Imanishi, T. Nakai, and Y. K. Konishi, Human autoantibodies that catalyze the hydrolysis of vasoactive intestinal polypeptide, Biochem (Life Sci. Adv.). 11: 173-177, 1992) and other groups have shown autoantibody mediated hydrolysis of DNA (Shuster, A. M., G. V. Gololobov, O. A. Kvashuk, A. E. Bogomolova, I. V. Smirnov, and A. G. Gabibov. DNA hydrolyzing autoantibodies, Science, 256: 665-667, 1992) and a cholinesterase activity associated with an anti-idiotypic antibody to anti-cholinesterase (Izadyar, L., A. Friboulet, M. H. Remy, A. Roseto, and D. Thomas, Monoclonal anti-idiotypic antibodies as functional internal images of enzymes active sites: Production of a catalytic antibody with a cholinesterase activity, Proc. Natl. Acad. Sci. USA., 90: 8876-8880, 1993).
Catalytic antibodies such as those discussed above are likely to cause more harm than non-catalytic antibodies. Clinical data suggest that certain autoimmune disorders may be caused by catalytic autoantibodies directed against nucleic acids, key regulatory proteins (i.e., insulin, glucagon, prolactin, VIP, substance P, blood clotting factors) and the cell surface receptors for these agents.
The presence of autoantibodies, however, is not an unequivocal sign of an autoimmune disease. This classification, in fact, is generally restricted to those cases in which the autoimmune reaction is the cause of tissue damage, either systemic or organ specific.
To date, autoimmune diseases have been linked to a defect of the immune system. Methods that facilitate the identification of patients who may be predisposed to autoimmune disorders are highly desirable so that early preventative treatments may be obtained.